1. Field of the Invention
The present invention relates to a method for the treatment of membranes to modify the properties thereof and to the membranes modified by such treatment. The membranes are industrially applicable in the field of ultrafiltration.
2. Description of the Prior Art
Membranes are obtained from various types of polymers generally by phase inversion processes. Their structure is partly crystalline, partly amorphous. The cohesion between the molecular chains is generally achieved by hydrogen bonding. The porous structure is extremely divided or loose and is totally temperature stable only below the first transition point.
Because of the manufacturing techniques involved, membranes generally present a radical local charge effect. This is due to the fact that during coagulation, or more generally phase inversion, some chains are blocked in a non-crystalline structure; and radicals have their charge potential non-counterbalanced by an adjoining counter radical from an adjoining chain.
Due to this effect, rejection of molecules by membranes is greatly influenced by the relative isoelectric point of the molecule retained at the particular pH because of molecular interaction between a negatively charged membrane and negatively charged molecules, e.g. proteins rather than the size of the molecule.
What appears to be an advantage in terms of rejection is also a disadvantage in terms of membrane poisoning. In fact, when ultrafiltration or other membrane-related processes are used for the treatment of a complex medium (e.g. blood, molasses, sugar-cane juice, whey) or generally any natural medium containing a large spectrum of various molecules those which are repelled by a partly charged membrane. The molecules are always accompanied by, molecules which are countercharged and will fix onto the membrane to create membrane poisoning.
Any membrane is a polymeric structure derived from natural, artificial or synthetic polymers. The cohesion of the structure in the original polymer is due to chemical bonds. A portion of these bonds are broken before the formation of the membrane, a portion of these are not reconstituted in the non-crystalline region of the polymer which constitutes the membrane. For example, carboxyl groups (C.dbd.O) and amine groups (--NH.sub.2) will be present locally at the surface of the polymer chains. Even for membranes which are described as being totally neutral, at the molecular level part of the structure has local ion-exchange properties, or local strong electro-chemical properties.
It is an object of the present invention to provide a method for transforming the surface characteristics of the polymer of the membrane--both at the surface of the membrane as such and within the internal structure thereof--to block the electro-chemical interactions, to achieve an antipoisoning effect at the molecular level; and to maintain steady flux characteristics during long-term use of the membrane.
It is a futher object of this invention to provide a process whereby one type of membrane can be used as a matrix for further reticulation or modification of the porous structure to give a large variety of possible molecular weight cut-offs from the same membrane structure.
By the use of previously known processes, the modification of the porous structure of membranes by grafting is extremely difficult to achieve without modification of the original membrane matrix. To graft any molecule onto the polymeric structure on a uniform basis, it is necessary to achieve accessability inside the polymer for the molecule chosen as the grafting reagent. The conventional method previously used was to increase the temperature well above the glass transition point of the polymer, to give the molecular chains sufficient flexibility to achieve diffusion of the reactive species.
This process is, however, not suitable for use with membranes because the use of temperatures above the glass transition point would destabilize the very fine and delicate structure of the membrane.